1. Field of the Invention
The present invention generally relates to a vehicle brake control system. More particularly, the present invention relates to a vehicle brake control system that cooperatively controls a frictional braking system and a regenerative braking system to enhance the feel of the braking operation in a transient period in which the regenerative braking torque changes suddenly.
2. Background Information
A typical vehicle brake control device controls regenerative braking and frictional braking components to achieve a target braking torque in response to a braking operation or other driving state. Generally, the vehicle brake control device will operate the regenerative braking components to perform regenerative braking to attempt to achieve the desired braking torque. If the desired braking torque cannot be achieved with regenerative braking, the control device can operate the frictional braking components to supply additional braking torque which compensates for the shortfall in braking torque provided by the regenerative braking components. Therefore, since regenerative braking is used as much as possible, the use of frictional braking can be kept to a minimum. Accordingly, it may be possible to maximize or at least increase the amount of kinetic energy of the vehicle that is recovered as electrical energy due to regenerative braking while minimizing the loss of vehicle kinetic energy as heat due to frictional braking. As a result, energy efficiency, fuel consumption and electrical consumption can be enhanced.
An example of a frictional braking system is described in Japanese Laid-Open Patent Application No. 2009-154814. The frictional braking system includes a motorized power assist-type master cylinder which operates in response to a braking operation via a brake pedal or the like. The master cylinder converts the braking operation force to fluid pressure that is used to operate a frictional brake unit to generate a frictional braking torque. The frictional braking torque can be moderated by motorized power assist control of the master cylinder.
Accordingly, braking force control is performed to supply a target braking torque based on a braking operation or the like. The regenerative braking system supplies a regenerative braking torque, and the frictional braking system which is moderated by motorized power assist control supplies frictional braking torque to compensate for any braking torque shortfall.
The motorized power assist-type master cylinder uses a motorized booster piston to push in a primary piston of the master cylinder. As a result, fluid pressure fluctuation during the motorized power assist control described above can cause variations in braking operation force. These variations can include variations in brake pedal effort which can adversely affect the feel of the braking operation. Therefore, when it is necessary to vary the frictional braking torque in response to variations in the regenerative braking torque, the fluid pressure fluctuation varies the brake pedal effort. Accordingly, the feel of the braking operation is adversely affected.
However, the system described in Japanese Laid-Open Patent Application No. 2009-154814 performs operations in an attempt to mitigate this variation in brake pedal effort. Specifically, a spring is placed between the primary piston and the motorized booster piston. Hence, the elastic deformation of the spring prevents the force that accompanies fluid pressure fluctuation from being fully transmitted to the brake pedal. Through this configuration, the variation of brake pedal effort due to variations in the apportionment of braking torque between regenerative braking and frictional braking can be mitigated. Thus, adverse effects on the feel of the braking operation can be reduced.